This invention relates to friction materials for all applications, and more particularly to semi-metallic and sintered full metallic friction materials utilized in heavy duty brake assemblies.
Sintered full metallic friction material members are well known, as described in U.S. Pat. No. 3,647,033 to Klein and U.S. Pat. No. 3,698,526 to Berges. A prior art full metallic mixture contains 60 to 90 percent by weight metal powders, 5 to 30 percent by weight carbonaceous materials, and zero to 15 percent by weight mineral fillers and friction enhancers. The mixture is typically molded at room temperature under extremely high pressures, on the order of 30,000 to 70,000 PSI. The resulting piece is then sintered in accordance with well defined concepts of powdered metallurgy. The bonding of the friction material member is due solely to the metal matrix formed by sintering. The metal content must be high, 60 percent or more by weight, to provide sufficient metal to metal contact to fuse the powders into a matrix. It is also well known that the piece may be sintered directly to a metal backing plate, e.g., a brake shoe.
The advantage of sintered full metallic friction material members is that they can operate at high temperature, and, when sintered to a metal backing plate, do not detach under high temperature and load conditions. However, full metallics are very expensive to manufacture. A large press, capable of exerting pressures in excess of 70,000 PSI, is required to mold the material. Mold life is short due to the high pressure molding. During the sintering process, the material has a tendancy to "bulge" or "edge crack"; and thus, these pieces must be discarded. In use, full metallic members tend to be ineffective when cold, structurally brittle and cause high opposing surface wear, often grooving the opposing surface, due to the member's high metal content.
Semi-metallic friction material members are also well known, as described in U.S. Pat. No. 3,647,033 to Klein and U.S. Pat. No. 3,434,998 to Aldrich. A conventional semi-metallic mixture contains 50 to 80 percent by weight metal fibers and powders, 10 to 20 percent by weight carbonaceous material, 7 to 20 percent by weight inorganic friction enhancers, zero to 5 percent by weight organic friction enhancers, and 5-15 percent by weight organic resin. The mixture is molded and the resin cured by the application of temperature, pressure and/or catalyst depending upon the particular resin used. The resulting pad is attached to a backing plate with an organic adhesive.
The principal differences between semi-metallics and full metallics is that: (1) the structural bonding of semi-metallics is due solely to the resin, while bonding in full metallics is due solely to the sintered metal matrix; (2) semi-metallics have considerably less percent by weight of metal particles than full metallics and typically contain metal fibers in addition to metal powders; (3) semi-metallics generally have a higher percent by weight of inorganic friction enhancers; and (4) typically contain organic friction enhancers (e.g., tire buffings), while full metallics, for the most part, do not contain organic friction enhancers (for the reason that organic materials will carbonize during sintering, reducing the density of the final pad and interfering with fusing of the metal powders).
The advantage of semi-metallics is that they are considerably cheaper to manufacture than full metallics, due to the much lower molding pressures and increased mold life, elimination of the sintering process, lower material costs, and a significant reduction in waste (no bulging or edge cracking with semi-metallics). However, in heavy duty applications, where operating temperatures often exceed 600.degree. F., the resin and the attaching adhesive tend to break down. The result is loss of friction, excessive wear of the friction material, detachment of the friction member from its backing plate, and in some instances flaming (the decomposing resin produces volitile gases which may ignite). A further disadvantage of semi-metallics is that the green (new) performance is not as good as full metallics, thus requiring a longer break-in period.
It would be desirable to have a friction material member that is relatively inexpensive to manufacture, performs well under high temperature and load conditions, does not detach or flame, has good green performance, and has greater resiliance.